1. Field of the Invention
The present invention relates to high efficiency turbomachinery and the integration of this turbomachinery into high efficiency thermodynamic cycles. In particular the Brayton and the Modified Ericsson cycles. This invention also includes the design of a high temperature tip-turbine driven centrifugal compressor that integrates well into Brayton and Modified Ericsson cycles.
2. Description of Related Art
The subject invention pertains to the design, the selection of high efficiency turbomachinery, and the integration of this turbomachinery into high efficiency thermodynamic cycles for: (1) power generation in space or on earth; (2) drive motors for vehicles, ships, trains and other modes of transportation; and (3) refrigeration applications such as helium liquefication for superconductivity, cryogenic fluid production, cooling of computers and electronic equipment, and air-conditioning. Current thermodynamic cycles being used for these applications include the Stirling, Rankine, Otto, Diesel and Brayton. Although the Ericsson cycle is often considered, it is usually abandoned because the added complexity of the turbomachinery and the need for many additional components, discourages its use. All these cycles except for the Ericsson cycle, have been highly developed over the past century and are widely used today. The Ericsson cycle, however, remains attractive because it can, like the Stirling, ideally achieve Carnot efficiencies when operated between given upper and lower temperature limits. The Carnot cycle is the most efficient cycle conceivable and forms a basis of comparison to other cycles. In practice, however, a Carnot engine is considered impractical because its size-to-power ratio is unfavorable when compared to modern engines. The Stirling cycle uses positive displacement type of machinery which has size limitations. The Modified Ericsson cycle, however, can use both, positive displacement for lower power levels, and high speed rotating axial and centrifugal flow type machinery at the higher power levels. The high speed rotating machinery has higher efficiency capability and higher power-to-weight ratios than the positive displacement type.
The Modified Ericsson approximates the Ideal Ericsson isothermal compression by using multiple stages of compression, with intercooling between stages, and the isothermal expansion by using multi-turbine stages, with reheat between stages. Therefore, many low pressure stages are beneficial, and ideally, the Modified Ericsson cycle approaches Carnot efficiencies with infinite number of stages as the pressure ratio per stage approaches one (1). Basically, the Modified Ericsson cycle is and expansion of the regenerative Brayton cycle, however, the specific power (net power/weight flow rate) increases proportionately as stages are added, without the need for additional regenerators. The regenerator is a critical component of a regenerative cycle because of its large size and high regeneration efficiency requirements, and any reduction to its size is highly desirable.
In summary, the significant advantages of Modified Ericsson engines over that of regenerative and nonregenerative Brayton, in addition to higher efficiency, is the increased specific power. For a given power, the higher specific power of the Modified Ericsson engine reduces the gas flow rate requirement proportionately and in turn the component sizes and weights.
The subject invention also pertains to the design of a high temperature tip-turbine driven compressor than can incorporate transpiration cooled turbine blades. The turbine blades or entire rotor could also be fabricated from a high temperature ceramic material such as silicon carbide, silicon nitride etc. The design also locates the bearing in an ambient temperature environment and eliminates heat-soak-back and oil lubrication coking problems.
Applications for turbocompressors of this type design include gas turbine and refrigeration power cycles that demand the highest turbine and compressor efficiencies possible with modest pressure rise requirements such as regenerative Brayton and Modified Ericsson cycles. Automotive superchargers is another application where moderate pressure rise compressors and high temperature turbines are required.
Most current gas turbines with cooled blades, such as aircraft derivative commercial type LM2500, 5000, and 6000, rely on blade root convection cooling using compressor discharge air. Such designs have been very successful and are widely used for commercial power plants. These designs, however, require very advanced technology and take years to develope with very large investment demands.
The subject design is considered more rugged, simpler to design and develop, and adapts well to land based power generation and refrigeration applications. Also, the subject design is better suited for ceramic application because the blades are bigger, and have large leading and trailing edges for ease of casting. In addition, the rotor tip-speeds and resulting stresses do not have to be as high as aircraft type designs since the need for light weight components is not critical for land based units.
Other benefits of the ruggedness of the subject design could be applied to coal burning and geothermal power plants where particulates in the gas can cause severe blade erosion of the leading and trailing edges of turbine and compressor blades with small radii and thus result in rapid loss of power plant efficiency. The use of large rugged thick ceramic blading of the subject design could extend replacement time of such units considerably.